This invention relates to a TTL (Transistor Transistor Logic) circuit in which the output level is maintained at the "H" level immediately after the power supply is turned on.
In a facsimile machine of a thermosensitive printer type, the enable period of the thermal heads of the printer is usually controlled by the output of a gate circuit called a "set gate", etc. A driving circuit for the thermal heads is illustrated in FIG. 1. In FIG. 1, SG indicates a set gate circuit, INV indicates an inverter circuit, DA1 through DAm indicate data latch circuits, OG1 through OGm indicate output gate circuits with NAND logic, TH1 through THm indicate thermal heads, and RL1 through RLm indicate resistors for current limiting. The set gate circuit SG is a TTL logic circuit which receives an external set input SET indicating whether the operation can be effected, and generates an output signal having the same logic level as the set signal. The inverter circuit INV inverts the output of the set gate circuit SG, and the output of the inverter is supplied to all of the output gate circuits OG1 through OGm.
When the set input SET is "H", the output of the inverter circuit INV is "L", all of the outputs of the output gate circuits OG1 through OGm are "H" and no current flows through any of the thermal heads TH1 through THm. On the other hand, when the set input SET is "L", the output of the inverter circuit INV is "H" and, when the output of the corresponding data latch circuit is "L", the output of each of the output gate circuits OG1 through OGm is "H". When the output of the corresponding data latch circuit is "H", the output of each of the output gate circuits OG1 through OGm is "L". When the output of an output gate circuit OGi is "L", current flows through a thermal head THi and a character corresponding to the output data of the latch circuit is printed.
In the driver circuit of FIG. 1, there may be the following problem immediately after the power supply is turned on. The set input SET should be kept at "H" until the printed data is input into the latch circuits DA1 through DAm and the printing operation is instructed. However, in a transition condition, such as when turning on the power supply, the set input SET does not settle into either a "H" or "L" state. Thus, it is not certain if the output of the set gate circuit SG will become "H" or "L" i.e., it may become "H" or "L" randomly. If the output of the set gate SG becomes "L", then the output of the inverter circuit INV becomes "H". In this condition, if any one of the data outputs of the latch circuits DA1 through DAm is "H", then the output of the corresponding output gate circuit becomes "L" and a current flows through the corresponding thermal head. Since the current flowing through the terminal head is considerably large, even several milliseconds of current may result in printing. Thus, undesirable stain-like printing may take place on the paper in the transition period when turning on the power supply.